Why is the shading off?
It is hard to tell conclusively without looking at your mesh.
Possible reasons for this kind of shading artifact are:

You may have a non-planar quad. (where not all the vertices that make up the quad are on the same plane)
You may have an n-gon. (despite from the screenshot it looks like a quad)

In either case adding a Weighted Normal modifier to your object should fix the problem.  However if you would like to go a bit deeper into this I would suggest first identifying the issue. (you can share your blend file here, we can take a look)

If the issue is having a non-planar face (quad), there is a Flatten command. (you can go into edit mode and press F3 to search.)
If the issue is having an n-gon, selecting the face in edit mode should tell you in the status bar how many vertices it has.  Probably there are two vertices very close to each other due to boolean operation.